Seismic design of new load-bearing unreinforced masonry buildings overlying open plan basements employing structural steelwork plane frames

Abstract

At first glance, the Maltese Islands are seen to be in a region of low seismic activity. However, the geographical location of the islands puts them in the vicinity of 13 active volcanoes and other highly seismically-active regions within the Mediterranean Basin, meaning that it would be irresponsible to dismiss the seismic hazard as negligible. From past activity, it has been concluded that the average tremor to strike the islands is that of a 3.5 magnitude on the Richter scale, which occurs infrequently. 1693 saw the Maltese Islands experience the worst earthquake to date with a magnitude of 7.4. The EU seismic design code (Eurocode 8) states that typical earthquakes responsible for structural collapses have a return period of 475 years. Statistically, therefore, it would not be unreasonable to expect an earthquake of a similar magnitude to hit Malta within the next 150 years. When it comes to low-rise load-bearing housing design in Malta, current practice relies on the use of masonry walls to support the structure at basement level. The seismic vulnerability of these buildings increases when the basement of the building is of a clear span due to its proposed use, most often for car parking. Previous studies have shown that local masonry buildings of similar construction built on rock can achieve a safety height limit of three storeys and be able to resist typical local seismic conditions (Galdes, 2013). This method of construction is not able to withstand tremors of high magnitudes or allow for taller building construction, making local load-bearing unreinforced masonry (URM) buildings particularly vulnerable to high risks of collapse during strong seismic events. In fact, such a building may be fittingly compared to a 'house on stilts', with very little sway resistance when built in isolation and not forming part of an aggregate. The critical issue of concern, therefore, is one of limited sway stiffness. The masonry basement construction between party-walls constitutes a low sway resisting system and will, therefore, suffer when experiencing the seismic loading especially in the transverse direction. The structural rigidity of the building can be improved by considering a different form of construction and by using a more ductile structural material within the basement. Incorporating a moment resisting frame of structural steelwork at the basement level to support the rest of the building could theoretically provide an increased structural sway stiffness, which would absorb and dampen the considerable lateral deformation without sustaining serious structural damage. The results obtained from this study show that there is scope for utilizing such sway resisting systems within the basement levels of URM buildings in order to enhance their seismic resistance during an earthquake event.